1. Technical Field
The present invention relates to a bone screw for use in correcting damage or deformation to a bone or group of bones in a body.
2. Description of the Prior Art
Bone screws are known in the medical profession. Typical uses for a bone screw are to treat a bone fracture, to attach a corrective device to parts of a fractured bone in the area adjacent to the fracture, or to attach a corrective device to a group of bones, such as a spinal
U.S. Pat. Nos. 2,121,193 and 2,570,465 each disclose a bone screw which has a threaded portion for engaging a cancellous portion of a bone. The threaded portion has a trailing face extending in a direction which is substantially perpendicular to the longitudinal axis of the bone screw. A bone screw disclosed in U.S. Pat. No. 4,175,555 has two threads of different pitch distances for engaging bone tissue. Each of the two threads has a face, which is substantially perpendicular to the longitudinal axis of the bone screw, for applying a clamping force to fractured parts of a bone as the bone screw is advanced.
U.S. Pat. No. 3,997,138 discloses another bone screw for securing rods of a corrective device to a vertebra. The bone screw includes two threaded portions and a boss integrally formed on the bone screw. The boss is located between the two threaded portions. The boss maintains the rods spaced away from the vertebra. A washer having projections for extending into the vertebra is located between the boss and the vertebra to aid in stabilizing the bone screw in a cancellous bone.
A bone screw similar to the bone screw disclosed in U.S. Pat. No. 2,121,193, referred to above, may be used to secure a corrective device to a vertebra. A first threaded portion of the bone screw is threaded into a cancellous portion of the vertebra. A nut is threaded onto a second threaded portion of the bone screw projecting from the vertebra. A corrective device is received on the second threaded portion of the bone screw and placed against the nut. A second nut is then threaded onto the second threaded portion of the bone screw. The second nut is tightened against the corrective device to apply a clamping force to the corrective device for preventing movement of the corrective device relative to the bone screw.
Due to load being transmitted from the corrective device to the bone screw in a direction perpendicular to the longitudinal central axis of the bone screw, the bone screw tends to move relative to the vertebra or the second threaded portion tends to bend relative to the first threaded portion. This bending of the bone screw could result in a crack developing in the surface of the bone screw and eventual failure. Also, if the corrective device is not seated properly between the nuts, a localized bending stress concentrates in the bone screw adjacent the nuts and the bone screw may break.
Furthermore, some fluids of the body are corrosive. If the surface of the bone screw pits due to the corrosive action of the fluids in an area of relatively high stress, cracks may initiate and eventual failure of the bone screw could occur. These fluids often flow to the highly stressed area of the bone screw from an opening in the vertebra for the bone screw.